x86: Check for APIC presence only if DEV_ATPIC is defined

We only attempt to gracefully handle absence of an APIC if "device
atpic" is defined in the kernel configuration.

Suggested by: kib
Reviewed by: jhb, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit deca0138dc659747c9edd4648690cdfdb01f58e8)